High titer, prostate specific antigen-specific human IgG production by hu-PBL-SCID mice immunized with antigen-mouse IgG2a complex-pulsed autologous dendritic cells

We report here that immunization of human PBMC reconstituted SCID mice (hu-PBL-SCID mice) with in vitro cultured autologous dendritic cells (DC) pulsed with prostate specific antigen (PSA) complexed to a PSA-specific mouse IgG2a (PSA-IgG2a) consistently and reproducibly stimulates PSA-specific human IgG production. On day 0, female PBMC were used to reconstitute SCID mice and to generate DC in vitro. DC cultures were pulsed with PSA or PSA-IgG2a on day 6. The previously reconstituted hu-PBL-SCID mice were immunized with either PSA-pulsed DC and PSA, PSA-IgG2a-pulsed DC and PSA-IgG2a, or additional PBMC and PSA-IgG2a on day 7. Mice immunized with PSA-IgG2a-pulsed DC had, on the average, up to 31.5 times greater PSA-specific IgG serum concentrations than control mice. Competition ELISA confirmed the PSA specificity of serum IgG. Immunoblot analysis suggested that sera IgG preferentially recognized conformational epitopes on PSA. Therefore, our results represent a major step toward cloning human tumor-associated Ag-specific human mAbs from hu-PBL-SCID mice. In addition, flow cytometry showed that PSA-pulsed DC express significantly more B7.1, B7.2, CD40, and MHC class II surface molecules than mock-treated DC, but PSA-IgG2a-pulsed DC only had significantly enhanced B7.2 surface expression. Interestingly, PSA-specific IgG responses were reproducibly stimulated by DC expressing more B7.2, a molecule associated with Th2-type immune deviation, but not by those expressing more B7.1 and CD40, molecules associated with Th1-type immune deviation. Thus, our results show that stimulation with either Ag or Ag complexed to mAb yields DC with different phenotypes and APC effector functions.